Contactless reading and writing of RFID tags (or radio-frequency identification tags or chips) can only be performed in an arrangement or network of antennas in certain optimal spatial conditions (position of the tag as close as possible to the reading antenna so that the projected surface is as large as possible) and interference conditions (electromagnetic waves other than those of the reading/writing antenna should not interfere).
These conditions are only partially fulfilled by known systems. This results in failures during the reading/writing phases with resulting errors in the management applications.
Already known from U.S. Pat. No. 6,700,547 is a remote interrogation portal using an electromagnetic field comprising an antenna coupled with a remote interrogator to detect when an object or an animal passes in the proximity of the antenna. The device antenna consists, in particular, of two co-planar loops made up of three portions—the top and the two side walls-forming a detection portal. Each part of the antenna is flattened against one side of the passage generating one of the three orthogonal directions of the 3D electromagnetic field produced. However, when the RFID tags are not presented according to one of the three orthogonal axes, the performance is reduced considerably and, in certain positions, it is difficult for the tag to be read by the antenna arrangement. On the other hand, the antennas are not used to their full capability because, since they are co-planar to the contours of the portal, half of the electromagnetic field (the part (1) outside the portal in reference to FIG. 1) is not used.
Also known from U.S. Pat. No. 4,798,175 is a portal for remote interrogation with an electromagnetic field. The device consists of a single antenna loop present on the top and side walls of the portal, the loop being reversed on one of the side walls so as to ensure that the RFID tag is read regardless of the direction in which it passes through the portal. This device is similar to that described above and has the same restrictions.
The antennas used in these two devices are relatively large since they closely follow the contour of the portal.
Also known from U.S. Pat. No. 6,696,954 or US 2002/0044096 is a network of antennas forming a portal or passage type detection area. More particularly, the network of antennas comprises a plurality of antenna loops arranged forming a rectangular network, the antennas transmitting/receiving electromagnetic signals having three-dimensional components. The antennas are also coupled with a processor by means of a filter and selective switches. In reference to FIG. 2 of US '954, the network of antennas comprises three loops, the first 30s, in the plane X-Y, surrounds the passage at the entry or exit thereof. The second and third loops 30v and 30h are placed diagonally (rotation respectively according to axis X and axis Y) along the detection area according to an angle substantially equal to 45° with the plane X-Y. Two independent antennas can also be added on the two side walls of the passage. The arrangement of antennas thus defines the parallelepiped, which is the volume of the detection area. However, the proposed device also has limitations, in particular because it allows an RFID tag to pass through the portal without forming an angle of less than 45°. Moreover, there are positions in which the tag is at an angle of 90° in relation to the two antennas 30v and 30h of the device, in particular when the tag is located at the center of the installation (where it is not subject to the influence of the lateral antennas) which is the most unfavorable scenario for reading or writing the tag. For this reason, the success rate for reading tags is not satisfactory for applications requiring a margin error of less then 1%.
Also known are systems generating a three-dimensional electromagnetic field by using phase changes in the various antennas of the device. This is particularly the case in WO 99/050780, which describes a contactless RFID tag reading device which powers the latter regardless of their position in the space of the induction coil of the circuit attached to the antenna. The device comprises three planar antennas arranged in three orthogonal planes so as to create three orthogonal fields according to the axes of a rectangular trihedron. These antennas are powered by currents in phase with the high-frequency carrier frequency and with amplitude modulated by time functions that can be sinusoidal functions of different frequencies.
Such systems have certain limitations, among others, the complexity of implementing a phase change system. Indeed, numerous parameters (R, L, C, material used, surface and shape of antennas and tags) render the system unstable. For example, for a system A comprising two antennas A1 and A2, each antenna has two electronic adaptation components, (A1, (a, b)) and (A2, (a′, b′))+2 components for changing the phase of the system made up of two antennas (A, (a″, b″)) giving a total of six interdependent parameters to be taken into account at the time of adjustment.
Also known from U.S. Pat. No. 3,689,885 is a system of interdependent antennas since, in practice, one antenna is used to supply energy to an RFID tag and another antenna is used for reading. This dependence requires correct tuning (matching impedance between the antenna and the reader in general at 5052 and 0°) of the antennas in relation to each other to allow the correct operation of the assembly. Such tuning is difficult to obtain, since modifying one antenna also modifies the tuning of the other antennas, this tuning then being obtained by dichotomy after numerous modifications of the various antennas. Since the electromagnetic antennas can “see” each other (which is to say they mutually influence each other), it is desirable to overcome the constraints by using independent antennas, which allow the tags to be powered and read simultaneously.
Also known from US 2003/0209601 is a system for monitoring articles by means of a plurality of stations, which have a network of antennas. Such networks comprise a plurality of vertical, horizontal or sidelong antennas, activated individually and sequentially. This network can be split into several zones (A, B, C, D), in which at the most two antennas (720; 730, 735; 740, 745; 750, 755) read the tags. However, this solution does not allow optimal reading of the RFID tags passing through the arrangement of antennas since, in a given location within the arrangement, it is possible for a tag to pass through it without doing so at a maximum angle of 45° in relation to one of the network antennas, this configuration enabling optimized reading of the tag.
WO 95/14938 discloses a device equipped with several antennas to enable an RFID tag to be identified, located and positioned. The aim of this invention is not to optimize the reading by guaranteeing a high success rate, but rather resides in the means for determining the position of the tag. It should be noted that this solution only uses a single reader for all the antennas, and that it implements antennas with complex geometry (antennas arranged in “C” or “L” formations, page 10).
It could therefore be advantageous to achieve an arrangement of antennas with simple geometry, which is to say flat antennas, minimizing complexity and costs by using a single reader.
Another problem to be dealt with is that, when the antenna surfaces are facing each other, the respective magnetic fields of each of the antennas interfere with one another, producing disturbance phenomena and therefore, incorrect operation. These phenomena, called “mutual magnetic coupling” are of physical nature and it is difficult to compensate for these coupling phenomena in the antennas used in equivalent devices.
U.S. Pat. No. 5,572,226 discloses a device with spherical antennas for contactless reading/writing of RFID tags, as well as a method of successively activating one of the antennas of the device. More particularly, the device switches from one antenna to another while controlling the waking state of the tags that are activated by antenna or the other. This solution depends to a great extent on the response supplied by the tags to the successive requests from each of the antennas. It does not make it possible to comprehend the problem of interference between antennas with substantially parallel surfaces.